Combustible gas sensors for the measurement of high levels of gases such as CO and H.sub.2 have been prepared in the past. These sensors operate by measuring a temperature differential developed when the combustible gas is catalytically reacted with oxygen at the sensor. Previous devices of this type are prone to a number of disadvantages: Rapid deactivation of the catalyst at high temperature results in an output signal of decreasing magnitude. Deterioration of the catalytic coating causes a loss in output accuracy with the passage of time. Low levels of SO.sub.2 poison the catalytic coating and lead to interference if this gas is present.
These disadvantages of the prior art devices combine to limit the usefulness and applicability of catalytic combustible gas sensors. For example, in the monitoring of boiler flue gas to permit control of the effluent to a required level, the combustible gas sensor must operate in a stable manner over long periods of time, and must be insensitive to a variety of interfering gases such as SO.sub.2 and NO and other contaminants that might arise from the combustion of the used fuel. In practice, it is found that combustible gas sensors prepared using prior art methods will measure combustible gases, typically CO and H.sub.2, and give reasonable operating life if operated in the temperature range of 150.degree.-260.degree. C. (300.degree. to 500.degree. F.). However, in the presence of even low levels of SO.sub.2 (500 ppm), these sensors show a greatly reduced output due to poisoning of the catalytic surface. Some insensitivity to SO.sub.2 poisoning can be gained by using a higher operating temperture, but this results in short sensor life. Also, a high operating temperature can lead to a decreased measuring span due to activity of the reference junction and erratic sensor output in the presence of SO.sub.2 and NO.